The present invention relates to a wind power generating system which supplies electric power to an electric power system.
In an AC excitation type generator or doubly-fed induction generator used in a power plant, a rotor winding is AC-excited at a slip frequency (difference between a system frequency and a rotation frequency) by means of a converter, so that voltage generated on the side of stator can be made to be equal to the system frequency by excitation of the rotor. There are merits that the excitation frequency (slip frequency) of the rotor can be changed to thereby vary the rotational number of a windmill and the capacity of a power converter can be made smaller as compared with the capacity of the generator.
However, low-order harmonics components (for example, 5th- and 7th-order) are contained in stator voltage depending on the slot structure of the stator and the rotor of the doubly-fed induction generator. Current of the converter on the grid-side is distorted due to the low-order harmonics components of the stator voltage. At this time, when a current control response is not raised sufficiently, it is apprehended that control of the converter spreads or increases the current distortion.
JP-A-2008-79383 discloses that a filter is inserted for control of current of a converter on the generator-side, although the filter is used to correct output of the current control system and there is no effect of eliminating the current distortion entering the current control system.